High-strength austenitic stainless steel for high-pressure hydrogen gas

ABSTRACT

There is provided an austenitic stainless steel for high-pressure hydrogen gas consisting, by mass percent, of C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more to less than 7%, Cr: 15 to 30%, Ni: 10% or more to less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, and at least one kind of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%, the balance being Fe and impurities, wherein in the impurities, the P content is 0.050% or less and the S content is 0.050% or less, the tensile strength is 800 MPa or higher, the grain size number (ASTM E112) is No. 8 or higher, and alloy carbo-nitrides having a maximum diameter of 50 to 1000 nm are contained in the number of 0.4/μm2 or larger in cross section observation.

This application is a Continuation of U.S. Ser. No. 14/007,992 filed onSep. 27, 2013, which is a national phase of PCT/JP2012/057001 filed onMar. 19, 2012, which is published as WO2012/132992 on Oct. 4, 2012.

TECHNICAL FIELD

The present invention relates to a high-strength stainless steel forhigh-pressure hydrogen gas, which has a tensile strength of 800 MPa orhigher, and has excellent mechanical properties in a high-pressurehydrogen gas environment.

BACKGROUND ART

In recent years, the development of fuel-cell vehicles that run usinghydrogen as the fuel and researches on practical hydrogen stations forsupplying hydrogen to fuel-cell vehicles have been advanced. A stainlesssteel is one of candidate materials used for these applications; still,in a high-pressure hydrogen gas environment, the stainless steel may besusceptible to embrittlement caused by hydrogen gas (hydrogenenvironment embrittlement). In accordance with the Exemplified Standardsof Compressed Hydrogen Vehicle Container stipulated in the High PressureGas Safety Act, the use of austenitic SUS316L is approved as a stainlesssteel that is not susceptible to hydrogen embrittlement.

In consideration of the necessity for reduced weight of fuel-cellvehicle and for high-pressure operation of hydrogen station, however,for a stainless steel used for a container and a pipe, there has been aneed for stainless steel that has a strength higher than that of theexisting SUS316L, especially has a tensile strength of 800 MPa or higherand is not susceptible to hydrogen environment embrittlement in ahydrogen gas environment. That is, assuming the use of high-pressurehydrogen of about 70 MPa, it is estimated that the SUS316L requires apipe and container to have a wall thickness of 20 mm or larger, whichleads to a significant increase in empty vehicle weight, so that higherstrength of steel is indispensable.

As a method for enhancing the strength of steel, cold rolling can becited as a typical method. Patent Document 1 gives a descriptionconcerning the cold rolling and the hydrogen environment embrittlementproperty of austenitic stainless steel.

As means for strengthening the austenitic stainless steel and improvingthe hydrogen embrittlement property of the austenitic stainless steelwithout relying on strengthening by cold rolling, Patent Documents 2 and3 propose high-strength stainless steels for high-pressure hydrogen gas,in which precipitation strengthening by means of fine nitrides isutilized.

Patent Document 2 proposes a high-strength austenitic stainless steel inwhich 7 to 30% of Mn, 15 to 22% of Cr, and 5 to 20% of Ni are containedas principal components, and Patent Document 3 proposes a high-strengthaustenitic stainless steel in which 3 to 30% of Mn, more than 22% to 30%or less of Cr, and 17 to 20% of Ni are contained as principalcomponents. These Documents indicate that a tensile strength of 800 MPaor higher can be realized in a state of solid solution heat treatment.

CITATION LIST Patent Document

[Patent Document 1] WO 2004/111285

[Patent Document 2] WO 2004/083477

[Patent Document 3] WO 2004/083476

SUMMARY OF INVENTION Technical Problem

In Patent Document 1, the influence of cold rolling on the hydrogenenvironment embrittlement has also been studied for SUS316L, in which itis verified that the cold rolling in the reduction of area of 30% orless does not have a great influence on the hydrogen environmentembrittlement property, indicating the possibility that a tensilestrength of about 800 MPa can be realized by cold rolling in thereduction of area of 20 to 30%. However, the high-strength austeniticstainless steel has a problem of the decrease in elongation and inhydrogen environment embrittlement property by cold rolling. Theinvention described in Patent Document 1 discloses, as measures againstthis problem, a technique in which cold rolling is performed at two ormore stages, and by performing cold rolling in different rollingdirections, the decrease in hydrogen environment embrittlement propertyand the decrease in elongation are restrained; however, the applicationof this invention inevitably requires considerably complicated coldrolling.

Further, in the case where a cold-rolled material is welded, localsoftening may be caused by welding heat affect. Therefore, it isdifficult to join the materials by a welded joint, and the joint of thematerials is restricted to a mechanical joint. To reduce the weight offuel-cell vehicle or to streamline the piping system for the hydrogenstation, there has been a strong need for a stainless steel that has ahigh strength and has no problem even if being welded. In this case,such means for achieving strengthening by cold rolling is difficult toapply in some respects.

The austenitic stainless steels described in Patent Documents 2 and 3realize a high strength of 800 MPa or higher in a state after solidsolution heat treatment. However, in Patent Document 2, when the Mncontent is less than 7%, a sufficient hydrogen environment embrittlementproperty cannot be obtained, and a sufficient strength cannot berealized in a state of solid solution heat treatment. Also, in the steelrelating to Patent Document 3, both of Cr concentration and Niconcentration are considerably high, so that this steel has adisadvantage of considerably high alloy cost.

The austenitic stainless steel described in Patent Document 2 can beproduced at a somewhat low alloy cost as compared with the steeldescribed in Patent Document 3. Therefore, if the stainless steel can beused in high-pressure hydrogen applications even if the stainless steelhas low content of Mn of less than 7% as compared with Patent Document2, an advantage is brought about in industrial production, since thesteels of this Mn content range have been used conventionally inapplications such as the nuclear field, and a common ingot can be used.

The present invention has been made in view of the present situation,and accordingly an objective thereof is to provide an austeniticstainless steel that has a high strength such that the tensile strengthis 800 MPa or higher and is excellent in hydrogen environmentembrittlement property in the composition range of less than 7% of Mn,which austenitic stainless steel has not been realized in PatentDocument 2.

Solution to Problem

The present inventors conducted various studies to solve the problem,and resultantly obtained the findings described in items (a) to (d)below.

(a) By utilizing nitrogen as a solute element, the strength of stainlesssteel can be enhanced. However, the addition of a large amount ofnitrogen decreases the stacking fault energy, and therefore has anadverse influence such that the distortion at the deformation time islocalized, and the durability against hydrogen environment embrittlementis decreased.

(b) By making grains fine, the resistance to hydrogen environmentembrittlement of high-nitrogen steel can be enhanced. As a method formaking grains fine, there is a method in which by precipitating finealloy carbo-nitrides at the time of final solid solution heat treatment,the growth of grains is restrained by the pinning effect. In order toproduce fine carbo-nitrides and to make the grains of high-nitrogensteel fine, it is most effective to add V or Nb. However, in theconventional method, although V and Nb precipitate as nitrides, V and Nbagglomerate and coarsen because of a small amount of precipitatenucleus, so that the pinning effect cannot be achieved sufficiently.

(c) As a method for solving this problem, as production processinvolving solid solution heat treatment, cold rolling, and secondaryheat treatment is effective. In the initial solid solution heattreatment, the alloying elements are dissolved sufficiently. In the nextcold rolling step, distortion is given, whereby the amount ofprecipitate nucleus of carbo-nitrides precipitating at the time of thenext secondary heat treatment is increased, the carbo-nitrides areprecipitated finely, and the grains are made fine.

(d) That is, in an alloy system having a Mn content lower than that ofPatent Document 2, by performing cold rolling at an intermediate stageof two heat treatments, the precipitation of carbo-nitrides isstimulated, and by the resultant refinement effect of austenite grainsand the precipitation strengthening action due to the precipitationitself of carbo-nitrides, a high strength can be attained, and also theresistance to hydrogen environment embrittlement can be enhanced.

The present invention has been completed based on the findings, and thegists thereof are austenitic stainless steels for high-pressure hydrogengas described in items (1) to (3) below.

(1) An austenitic stainless steel for high-pressure hydrogen gasconsisting, by mass percent, of C: 0.10% or less, Si: 1.0% or less. Mn:3% or more to less than 7%, Cr: 15 to 30%, Ni: 10% or more to less than17%, Al: 0.10% or less. N: 0.10 to 0.50%, and at least one kind of V:0.01 to 1.0% and Nb: 0.01 to 0.50%, the balance being Fe and impurities,wherein in the impurities, the P content is 0.050% or less and the Scontent is 0.050% or less, the tensile strength is 800 MPa or higher,the grain size number (ASTM E112) is No. 8 or higher, and alloycarbo-nitrides having a maximum diameter of 50 to 1000 nm are containedin the number of 0.4/μm² or larger in cross section observation.

(2) An austenitic stainless steel for high-pressure hydrogen gasconsisting, by mass percent, of C: 0.10% or less, Si: 1.0% or less, Mn:3% or more to less than 7%, Cr: 15 to 30%. Ni: 10% or more to less than17%, Al: 0.10% or less. N: 0.10 to 0.50%, and at least one kind of V:0.010 to 1.0% and Nb: 0.01 to 0.50%, further containing one or morekinds of elements of at least one group selected from element groups ofa first group to a fourth group described below, the balance being Feand impurities, wherein in the impurities, the P content is 0.050% onless and the S content is 0.050% or less, the tensile strength is 800MPa or higher, the grain size number (ASTM E112) is No. 8 or higher, andalloy carbo-nitrides having a maximum diameter of 50 to 1000 nm arecontained in the number of 0.4/μm² or larger in cross sectionobservation.

First group elements . . . Mo: 0.3 to 3.0% and W: 0.3 to 6.0%

Second group elements . . . Ti: 0.001 to 0.5%, Zr: 0.001 to 0.5%, Hf:0.001 to 0.3%, and Ta: 0.001 to 0.6%

Third group elements . . . B: 0.0001 to 0.020%, Cu: 0.3 to 5.0%, and Co:0.3 to 10.0%

Fourth group elements . . . Mg: 0.0001 to 0.0050%, Ca: 0.0001 to0.0050%, La: 0.0001 to 0.20%, Ce: 0.0001 to 0.20%, Y: 0.0001 to 0.40%,Sm: 0.0001 to 0.40%, Pr: 0.0001 to 0.40%, and Nd: 0.0001 to 0.50%

(3) The austenitic stainless steel for high-pressure hydrogen gasdescribed in item (1) or (2), wherein the austenitic stainless steel issubjected to solid solution heat treatment at a temperature of 1100 to1200° C. next being subjected to cold rolling in which the reduction ofarea is 20% or more, and thereafter is again subjected to heat treatmentin the temperature range of 900° C. or higher and lower than thesolution treatment temperature.

Advantageous Effect of Invention

According to the present invention, there can be provided ahigh-strength austenitic stainless steel that has a tensile strength of800 MPa or higher and is excellent in hydrogen environment embrittlementproperty in the composition region of less than 7% of Mn.

DESCRIPTION OF EMBODIMENT

The reasons for restricting the chemical composition and metalmicro-structure of a steel plate in the present invention are asfollows:

(A) Chemical Composition of Steel

The operational advantages of each component of steel and the preferablecontent of each component are described below. The symbol “%” concerningthe content of each element means “mass percent”.

C: 0.10% or Less

In the present invention, C (carbon) is not an element that is addedpositively. If the C content is more than 0.10%, carbides precipitate atthe gram boundaries, and exert an adverse influence on toughness and thelike. Therefore, the C content is restrained to 0.10% or less. The Ccontent is preferably 0.04% or less, further preferably 0.02% or less.The C content should be as low as possible. However, the extremereduction in C content leads to an increase in refining cost, so that itis desirable to make the C content 0.001% or more in practicalapplication.

Si: 1.0% or Less

If Si (silicon) is contained in large amounts, Si forms an intermetalliccompound with Ni, Cr, or the like, or promotes the formation of anintermetallic compound such as sigma phase, so that, in some cases, thehot workability is decreased remarkably. Therefore, the Si content is1.0% or less. Preferably, the Si content is 0.5% or less. The Si contentshould be as low as possible. However, considering the refining cost, itis desirable to make the Si content 0.01% or more.

Mn: 3% or More to Less than 7%

Mn (manganese) is an inexpensive austenite stabilizing element. In thesteel of the present invention, due to a proper combination with Cr, Ni,N, and the like, Mn contributes to the enhancement of strength and theimprovement in ductility and toughness. The present invention also hasan aim of finely precipitating carbo-nitrides and making the grainsfine. In the case where the amount of dissolved N is small, even if thesteel undergoes the later-described process consisting of solid solutionheat treatment, cold rolling, and secondary heat treatment,carbo-nitrides having a sufficient number density cannot beprecipitated, and it becomes difficult to enhance the strength due tofiner austenite grains. Therefore, 3% or more of Mn must be contained.If the Mn content is 7% or more, the technique described in PatentDocument 2 can be applied. Therefore, in the present invention, theupper limit of the Mn content is less than 7%. For these reasons, the Mncontent is specified so as to be 3% or more to less than 7%. Thepreferable lower limit of the Mn content is 4%. Also, the Mn content iseffective when being 6.5% or less, especially effective when being 6.2%or less.

Cr: 15 to 30%

Cr (chromium) is an essential component because it is an element forensuring corrosion resistance as a stainless steel. The Cr content mustbe 15% or more. However, if the Cr content is excessively high, coarsecarbides such as M₂₃C₆, which decrease the ductility and toughness, areeasily thrilled in large amounts. Therefore, the proper Cr content is 15to 30%. The Cr content is preferably 18 to 24%, further preferably 20 to23.5%.

Ni: 10% or More to Less than 17%

Ni (nickel) is added as an austenite stabilizing element. In the steelof the present invention, due to a proper combination with Cr, Mn, N,and the like, Ni contributes to the enhancement of strength and theimprovement in ductility and toughness. Therefore, the Ni content is 10%or more. However, if the Ni content is 17% or more, the effectsaturates, and the material cost increases. For these reasons, theproper Ni content is 10% or more to less than 17%. The Ni content ispreferably 11 to 15%, further preferably 11.5 to 13.5%.

Al: 0.10% or Less

Al (aluminum) is an important element as a deoxidizer. However, if theAl content is more than 0.10% and Al remains in large amounts, theformation of an intermetallic compound such as sigma phase is promoted.Therefore, to attain both of the strength and toughness intended by thepresent invention, the Al content must be restricted to 0.10% or less.In order to reliably achieve the deoxidizing effect, the Al content isdesirably 0.001% or more. The Al content is preferably 0.05% or less,further preferably 0.03% or less. In this description, Al meansso-called “sol. Al (acid soluble Al)”.

N: 0.10 to 0.50%

N (nitrogen) is the most important solid-solution strengthening element,and at the same time, in the present invention, makes the grains finedue to the formation of fine alloy carbo-nitrides, contributing to theenhancement of strength. To utilize N for the enhancement of strength,0.10% or more of N must be contained. However, if the N content is morethan 0.50%, coarse nitrides are formed, and therefore the mechanicalproperties such as toughness decrease. Therefore, the N content is 0.10to 0.50%. The lower limit of the N content is preferably 0.20%, furtherpreferably 030%.

V: 0.01 to 1.0% and/or Nb: 0.01 to 0.50%

V (vanadium) and Nb (niobium) are important elements in the steel of thepresent invention. To promote the formation of alloy carbo-nitrides andto contribute to finer grains, either one or both of V and Nb must becontained. For these purposes, 0.01% or more of V and/or Nb must becontained. On the other hand, even if more than 1.0% of V and/or morethan 0.50% of Nb are contained, the effect saturates, and the materialcost increases, so that the upper limits of the V content and the Nbcontent are 1.0% and 0.50%, respectively. The V content is preferably0.10 to 0.30%, and the Nb content is preferably 0.15 to 0.28%. Thecontaining of both of V and Nb is more elective.

P: 0.050% or Less

P (phosphorus), which is an impurity, is an element that exerts anadverse influence on the toughness and the like of steel. The P contentis 0.050% or less, and is preferably as low as possible. The P contentis preferably 0.025% or less, further preferably 0.018% or less.

S: 0.050% or Less

S (sulfur), which is an impurity, is an element that like P, exerts anadverse influence on the toughness and the like of steel. The S contentis 0.050% or less, and is preferably as low as possible. The S contentis preferably 0.010% or less, further preferably 0.005% or less.

The steel in accordance with the present invention has theabove-described chemical composition, and in the steel, the balanceconsists of Fe and impurities. The “impurities” in the “Fe andimpurities” mean components that mixed in on account of various factorsin the production process, including raw materials such as ore or scrap,when a steel is produced on an industrial scale, the components beingallowed to exist in the range such that they do not an adverse influenceon the present invention.

The steel in accordance with the present invention can contain, asnecessary, one or more kinds of components selected from at least onegroup of the first group to the fourth group described below. Hereunder,the components belonging to these groups are described.

The elements belonging to the first group are Mo and W. These elementshave a common operational advantage of stimulating the formation andstabilization of carbo-nitrides and contributing to solid-solutionstrengthening. The reasons for restricting the contents of theseelements are as described below.

Mo: 0.3 to 3.0%, W: 0.3 to 6.0%

Mo (molybdenum) and W (tungsten) have an effect of formingcarbo-nitrides and thereby making the grains fine, and also contributeto solid-solution strengthening. Either of these elements achieves theeffect when the content of each of these elements is 0.3% or more, sothat these elements can be contained as necessary. However, even ifthese elements are contained excessively, the effect saturates.Therefore, if these elements are contained, the contents thereof shouldbe as follows: Mo: 0.3 to 3.0%, and W: 0.3 to 6.0%.

The elements belonging to the second group are Ti, Zr, Hf, and Ta. Theseelements have a common operational advantage of stimulating theformation of carbo-nitrides.

Ti: 0.001 to 0.5%, Zr: 0.001 to 0.5%, Hf: 0.001 to 03%, Ta: 0.001 to0.6%

Ti (titanium), Zr (zirconium), Hf (hafnium), and Ta (tantalum), which,like V and Nb, have an effect of forming alloy carbo-nitrides andthereby making the grains fine, can be contained as necessary. Thiseffect can be achieved by containing 0.001% or more of each of theseelements. However, even if these elements are contained excessively, theeffect saturates. Therefore, the upper limits of the contents of theseelements are respectively as follows: 0.5%, Zr: 0.5%, Hf: 0.3%, and Ta:0.6%. The upper limits of contents of Ti and Zr are preferably 0.1%,further preferably 0.03%. The upper limit of the Hf content ispreferably 0.08%, further preferably 0.02%. The upper limit of the Tacontent is preferably 0.4%, further preferably 0.3%.

The elements belonging to the third group are B, Cu, and Co. Theseelements contribute to the enhancement of strength. The reasons forrestricting the contents of these elements are as described below.

B: 0.0001 to 0.020%

B (boron), which makes the precipitates line, and decrease the austenitegrain diameter, whereby increasing the strength, can be contained asnecessary. The effect thereof is achieved when the B content is 0.0001%or higher. On the other hand, if the B content is excessive, a compoundof low melting point is formed, and the hot workability may bedecreased. Therefore, the upper limit of the B content is 0.020%.

Cu: 0.3 to 5.0%, Co: 0.3 to 10.0%

Cu (copper) and Co (cobalt) are austenite stabilizing elements, andcontribute to the enhancement of strength due to solid-solutionstrengthening. Therefore, 0.3% or more or either one or both of theseelements can be contained as necessary. However, because of the balancebetween effect and material cost, the upper limits of the contents of Cuand Co are 5.0% and 10.0%, respectively.

The elements belonging to the fourth group are Mg, Ca, La, Ce, Y, Sm,Pr, and Nd. These elements have a common action for preventingsolidification cracking at the time of casting.

Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, La: 0.0001 to 0.2%, Ce:0.0001 to 0.20%, Y: 0.0001 to 0.40%, Sm: 0.0001 to 0.40%, Pr: 0.0001 to0.40%, and Nd: 0.0001 to 0.50%

Mg (magnesium) and Cu (calcium), and La (lanthanum), Ce (cerium), V(yttrium), Sm (samarium), Pr (praseodymium), and Nd (neodymium) amongtransition metals have an action for preventing solidification cracking,at the time of casting. Therefore, one or more kinds of these elementsmay be contained as necessary. The effect can be achieved by containing0.0001% or more of each a these elements. On the other hand, if theseelements are contained excessively, the hot workability decreases.Therefore, the upper limits of the contents of these elements are asfollows: Mg and Ca: 0.0050%, La and Ce: 0.20%, Y, Sm and Pr: 0.40%, andNd: 0.50%.

(B) Micro-Structure of the Steel

The nitrogen used in the present invention is effective in performingsolid-solution strengthening, but has an action such that the distortionat the time of deformation is localized try decreasing the stackingfault energy, and the durability against hydrogen environmentembrittlement is decreased. However, by decreasing the grain diameter,both of the enhancement of strength to 800 MPa or higher and theprevention of hydrogen environment embrittlement are enabled. In orderto prevent hydrogen environment embrittlement, the grain size number(ASTM E112) is No. 8 or higher, preferably No. 9 or higher, and furtherpreferably No. 10 or higher.

In order to make the grains fine, the pinning utilizing alloycarbo-nitrides is effective. To achieve this effect, alloycarbo-nitrides having a size of 50 to 1000 nm must be contained in thenumber of 0.4/μm² or larger in cross section observation. These alloycarbo-nitrides are those that contain Cr, V, Nb, Ta, and the like asprincipal components, and have a crystalline structure of Z phase, thatis, Cr(Nb, V)(C, N) or of MX type (M: Cr, V, Nb, Mo, W, Ta, and thelike, X: C, N). The alloy carbo-nitrides in the present invention arecarbo-nitrides scarcely containing Fe. Even if Fe is contained, theamount of Fe is 1 atom % or less. Also, the carbo-nitrides in thepresent invention include those in which the content of C (carbon) isextremely tow, that is, those consisting of nitrides.

(C) Production Method

In order to make grains fine as described in (B) and to precipitate finealloy carbo-nitrides having a desired number density, the ordinarymethod cannot be used. However, the steel of the present invention canbe produced by successively performing the solid solution heattreatment, cold rolling, and secondary heat treatment described below.

The first solid solution heat treatment must be performed at atemperature of 1000° C. or higher, preferably 1100° C. or higher, todissolve alloying elements sufficiently. However, if the solid solutionheat treatment temperature is higher than 1200° C., the grains arecoarsened extremely. Therefore, the upper limit of the solid solutionheat treatment temperature is 1200° C. Hereunder, for convenience, theheat treatment temperature in the solid solution heat treatment isreferred to as a “T1 temperature”.

In the solid solution heat treatment in accordance with the presentinvention, solution treatment of a degree necessary for precipitatingcarbo-nitrides in the later secondary heat treatment has only to beperformed, and all of the carbo-nitride forming elements need notnecessarily be dissolved. The steel material having been subjected tosolid solution heat treatment is preferably cooled rapidly from thesolid solution heat treatment temperature. In this case, water cooling(shower water cooling or dipping) is preferable.

Also, concerning the solid solution heat treatment, an independent solidsolution heat treatment step need not necessarily be provided. Byperforming rapid cooling after a process of hot working such as hotextrusion, the equivalent effect can be achieved. For example, rapidcooling has only to be performed after hot extrusion at about 1150° C.

Next, in order to increase the amount of precipitate nucleus ofcarbo-nitrides, cold rolling is performed at a cold rolling ratio suchthat the reduction of area is 20% or more. The upper limit of coldrolling ratio is not restricted especially. However, considering theworking ratio at the time when an ordinary member is subjected to coldrolling, 90% or less of cold rolling ratio is preferable. Finally, inorder to remove distortion caused by cold rolling and to making thegrains fine by precipitating fine carbo-nitrides, secondary heattreatment is perforated at a temperature lower than the T1 temperature.Hereunder, for convenience, the heat treatment temperature in thesecondary heat treatment is referred to as a “T2 temperature”.

The T2 temperature is less than the T1 temperature. In order to make thegrains finer, the upper limit of the T2 temperature is preferably made[T1 treatment temperature−20° C.], and further preferably made [T1treatment temperature−50° C.]. Specifically, the upper limit of the T2temperature is preferably made 1150° C., and further preferably made1080° C. On the other hand, the lower limit of the T2 temperature is900° C. because if the T2 temperature is lower than 900° C., coarse Crcarbides are formed, and therefore the micro-structure becomesnon-uniform.

EXAMPLES

In the following, the effects of the present invention are explainedbased on examples.

Fifty kilograms of each of stainless steels having the chemicalcompositions given in Table 1 was vacuum melted and hot-forged to form ablock having a thickness of 40 to 60 mm.

TABLE 1 Chemical composition (mass %, the balance being Fe) Steel C SiMn P S Ni Cr V Nb sol.Al N Others A 0.020 0.40 4.55 0.010 0.001 12.2522.50 0.20 0.20 0.020 0.32 The B 0.010 0.42 5.80 0.015 0.001 13.45 20.580.28 0.15 0.015 0.36 invention C 0.008 0.43 4.60 0.009 <0.001 12.5522.10 0.12 0.28 0.017 0.30 steel D 0.005 0.46 1.12 0.015 0.001 12.1918.31 0.08 0.05 0.016 0.20 E 0.015 0.45 5.80 0.018 <0.001 11.22 18.530.21 0.018 0.25 F 0.005 0.40 5.10 0.008 0.002 14.85 23.75 0.48 0.0250.45 G 0.050 0.35 6.84 0.020 <0.001 10.25 15.15 0.23 0.018 0.10 H 0.0550.36 4.51 0.009 0.001 10.85 17.85 0.05 0.22 0.022 0.12 I 0.033 0.65 3.100.015 0.003 16.2 28.65 0.65 0.045 0.42 J 0.025 0.45 4.75 0.008 0.00112.20 22.10 0.21 0.10 0.018 0.31 Mo: 2.10 K 0.021 0.43 4.55 0.010 0.00112.55 23.95 0.18 0.20 0.020 0.30 Ti: 0.022 L 0.009 0.43 5.10 0.012<0.001 11.80 20.22 0.10 0.15 0.028 0.31 B: 0.0030 M 0.019 0.46 5.010.009 0.001 12.05 23.15 0.19 0.21 0.019 0.32 Cu: 3.5 N 0.021 0.48 4.850.008 0.001 13.20 21.84 0.28 0.09 0.021 0.30 Ca: 0.0016 O 0.015 0.364.95 0.014 <0.001 12.96 22.01 0.22 0.20 0.020 0.30 Nd: 0.32 P 0.019 0.445.05 0.015 0.001 11.85 22.55 0.18 0.19 0.022 0.31 Mo: 1.95, Zr: 0.025 Q0.035 0.49 5.52 0.008 0.002 13.20 23.01 0.12 0.20 0.028 0.30 W: 4.01, B:0.0055 R 0.022 0.44 4.88 0.009 0.001 12.05 22.20 0.20 0.15 0.017 0.33Mo: 2.05, Mg: 0.0025 S 0.021 0.43 4.55 0.010 0.001 12.85 22.95 0.18 0.100.020 0.30 Ta: 0.20, Cu: 4.5 T 0.015 0.45 4.89 0.009 0.002 12.09 21.060.19 0.20 0.025 0.30 B: 0.015, Ca: 0.0025 U 0.011 0.44 4.86 0.010 0.00112.08 20.85 0.15 0.19 0.020 0.38 B: 0.015, Mg: 0.0041 V 0.015 0.45 5.090.012 0.001 12.04 21.06 0.19 0.20 0.021 0.39 Cu: 4.8, Ca: 0.0035 W 0.0090.48 4.86 0.008 <0.001 12.07 20.96 0.26 0.09 0.019 0.36 Mo: 2.15, Ti:0.010, B: 0.0025 X 0.010 0.47 4.99 0.011 0.001 12.51 21.48 0.21 0.150.015 0.32 Mo: 1.95, Ti: 0.015, Cu: 3.7 Y 0.016 0.47 5.21 0.011 0.00112.25 21.59 0.24 0.18 0.018 0.30 Mo: 2.15, Zr: 0.045, Ca: 0.0020 Z 0.0200.49 5.56 0.012 0.002 13.16 23.08 0.27 0.20 0.018 0.33 Ta: 0.21, Cu:4.2, Mg: 0.0035 1 0.015 0.46 4.95 0.015 <0.001 12.95 22.98 0.23 0.140.016 0.30 Mo: 2.85, Ti: 0.010, Cu: 3.5, La: 0.10 2 0.010 0.41 5.250.009 0.001 13.01 21.91 0.21 0.15 0.021 0.31 Mo: 3.01, Ti: 0.009, Cu:3.0, Y: 0.11 3 0.011 0.45 4.91 0.008 <0.001 13.25 22.05 0.20 0.13 0.0200.30 Mo: 2.95, Ti: 0.012, Cu: 3.4, Pr: 0.11 4 0.035 0.44 2.05* 0.0080.001 12.51 21.95 0.23 0.019 0.06* Comparative 5 0.009 0.46 5.01 0.0070.001  9.02* 22.10 0.25 0.018 0.31 steel 6 0.012 0.49 5.22 0.012 0.00112.35 30.55* 0.23 0.019 0.30 7 0.009 0.42 5.01 0.009 0.002 12.14 21.960.16 0.023 0.05* *shown out of scope of the invention steel.

Thereafter, the block was hot-rolled to a predetermined thickness, andwas subjected to one-hour solid solution heat treatment, cold rolling,and one-hour secondary heat treatment, whereby an 8-mm thick platematerial was formed. In Table 2, the solid solution heat treatmenttemperature (T1 temperature) of each test No. is expressed by T1(° C.),and the secondary heat treatment temperature (T2 temperature) thereof isexpressed by T2(° C.). The cold rolling ratio of each test No. is alsoshown in Table 2.

TABLE 2 Cold Number of Relative Test T1 rolling T2 Grain sizecarbo-nitrides TS rupture No. Steel (° C.) ratio (° C.) No. (×10/25 μm²)(MPa) elongation (%) 1 A 1100 25  900 10.2 35 826 96 The 2 A 1100 401000 10.6 42 814 98 invention 3 A 1100 50  950 11.0 49 822 103 4 A 115050 1050 10.5 55 828 101 5 A 1150 25 1050  9.1 33 819 92 6 A 1100 40 1050 9.6 41 821 93 7 A 1200 25 1150  8.2 28 815 82 8 B 1100 25 1000 10.3 29808 96 9 C 1100 25 1000 10.5 27 805 98 10 D 1100 25 1000 10.3 20 812 9211 E 1100 25 1000 10.4 23 809 93 12 F 1100 25 1000 10.3 82 865 92 13 G1100 25 1000 10.1 12 805 85 14 H 1100 25 1000 10.5 15 812 86 15 I 110025 1000 10.3 65 854 89 16 J 1100 25 1000 10.2 31 812 100 17 K 1100 251000 10.3 25 811 101 18 L 1100 25 1000 10.3 29 808 98 19 M 1100 25 100010.2 27 809 99 20 N 1100 25 1000 10.2 28 814 100 21 O 1100 25 1000 10.328 830 99 22 P 1100 25 1000 10.3 25 815 104 23 Q 1100 25 1000 10.2 24806 99 24 R 1100 25 1000 10.4 27 803 99 25 S 1100 25 1000 10.3 34 830 9826 T 1100 25 1000 10.2 31 809 96 27 U 1100 25 1000 10.2 58 825 95 28 V1100 25 1000 10.2 65 842 95 29 W 1100 25 1000 10.3 52 815 96 30 X 110025 1000 10.4 25 805 105 31 Y 1100 25 1000 10.4 24 808 101 32 Z 1100 251000 10.3 31 831 98 33 1 1100 25 1000 10.3 33 812 99 34 2 1000 25  95010.9 25 813 101 35 3 1000 25  950 10.9 24 822 102 36 A 1250** 40 1000 7.5* 38 802 65 Comparative 37 A  950** 40 1010  7.2*  0.3* 666* 63 38 A1100  0** 1000  6.7*  0.2* 654* 53 39 A 1100 15** 1000  7.2*  0.5* 704*58 40 A 1100 25 1100**  7.8* 31 805 75 41 A 1100 40  850  7.5*  0.3*688* 73 42 4 1100 25 1000  7.3*  0.1* 581* 74 43 5 1100 25 1000 10.5 22813 55 44 4 1100 25 1000 10.6 28 802 45 45 7 1100 25 1000  7.7*  0.2*560* 73 *shows out of scope of the invention steel. **shows out of scopeof the invention method.

A specimen was sampled and embedded with a resin so that the crosssection perpendicular to the rolling direction of the plate material canbe observed, and after electrolytic etching, the grain size number (inconformity to ASTM E112) was measured. Also, similarly, by using a resinembedding material in the cross section direction, the number ofprecipitates was measured by the observation under an electronmicroscope using the extraction replica method. A region of 25 μm² wasobserved at ×10,000 magnification in ten visual fields, and precipitateshaving a size of 50 to 1000 nm were measured. The precipitates measuredin examples were carbo-nitrides of Z phase of rhombic structurecontaining Cr, V, Nb, C, N, and the like, or of MX type of tetragonalstructure containing Cr, Nb, V, C, N, and the like.

A round-bar tensile test specimen having a diameter of 3 mm in itsparallel part was sampled in the longitudinal direction of the platematerial, and a tensile test was conducted at a strain rate of 3×10⁻⁶/sin the atmosphere at normal temperature or in high-pressure hydrogen gasof 85 MPa at normal temperature to measure tensile strength (TS) andrupture elongation. Since hydrogen has a remarkable influence on thedecrease in ductility, the ratio of rupture elongation in hydrogen torupture elongation in the atmosphere was made a relative ruptureelongation, and it was interpreted that if the relative ruptureelongation is 80% or more, preferably 90% or more, the decrease inductility caused by hydrogen is slight, and the resistance to hydrogenenvironment embrittlement is excellent.

The strain rate of 3×10⁻⁶/s in the above-described tensile test isconsiderably lower than the strain rate of 10⁻⁴/s in the tensile test inthe high-pressure hydrogen gas environment, which has been used in theconventional documents. The reason for this is that in the recentevaluation standards in durability evaluation against hydrogenenvironment embrittlement, the evaluation test at a very low strainrate, in which the hydrogen environment embrittlement susceptibility ofaustenitic stainless steel becomes higher, is recommended.

Table 2 summarized the grain size number, the number of carbo-nitrides,tensile strength (TS), and relative rupture elongation of steel beingtested. Test Nos. 1 to 35 are example embodiments of the presentinvention, in which the grain size number was No. 8 or higher, asufficient number of carbo-nitrides were precipitated, the TS was 800MPa or higher, and the relative rupture elongation was also 80% or more,a sufficient resistance to hydrogen environment embrittlement beingattained.

Test Nos. 36 to 41 are comparative examples. In test No. 36, the solidsolution heat treatment temperature T1 was too high, the grains werecoarsened, and the resistance to hydrogen environment embrittlement waspoor. In test No. 37, the solid solution heat treatment temperature T1was too low, the number density of carbo-nitrides was low, the grainswere coarsened, and the resistance to hydrogen environment embrittlementwas poor. In test Nos. 38 and 39, the cold rolling ratio was low, theprecipitation number of carbo-nitrides was insufficient, the grains werecoarsened, and the resistance to hydrogen environment embrittlement waspoor. In test No. 40, the secondary heat treatment temperature T2 wastoo high, the grains were coarsened, and the resistance to hydrogenenvironment embrittlement was poor. In test No. 41, the final solidsolution heat treatment temperature T2 was too low, the number densityof carbo-nitrides was low, the grains were coarsened, and the resistanceto hydrogen environment embrittlement was poor.

Test Nos. 42 to 45 are comparative examples, in which the chemicalcomposition of steel material was out of the range of the presentinvention. In test No. 42, the Mn content was too low, and resultantly N(nitrogen) could not be contained sufficiently, the grains werecoarsened, the strength was low, and the resistance to hydrogenenvironment embrittlement was poor. In test No. 43, the Ni content waslow, δ ferrite was formed, and the resistance to hydrogen environmentembrittlement was poor. In test No. 44, the Cr content was high, coarseCr carbides were formed, and the resistance to hydrogen environmentembrittlement was poor. In test No. 45, the N (nitrogen) content waslow, the grains were coarsened, the strength was low, and the resistanceto hydrogen environment embrittlement was poor.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention even anaustenitic stainless steel containing less than 7% of Mn can be made ahigh-strength steel excellent in hydrogen environment embrittlementproperty by interposing a cold rolling step between two heat treatments,and therefore can be used for pipes and containers for high-pressurehydrogen gas.

The invention claimed is:
 1. A method of manufacturing an austeniticstainless steel for hydrogen gas, wherein the tensile strength is 800MPa or higher, a relative rupture elongation is 80% or more, the grainsize number in accordance with ASTM E112 is No. 8 or higher, and alloycarbo-nitrides having a maximum diameter of 50 to 1000 nm are containedin the number of 0.4/μm² or larger in cross section observation,comprising the following steps (a) to (d): (a) preparing an austeniticstainless steel that has the chemical composition comprising, by masspercent, C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more to less than7%, Cr: 15 to 24%, Ni: 10 to 15%, Al: 0.001 to 0.10%, N: 0.10 to 0.50%,and V: 0.01 to 1.0%, the balance being Fe and impurities, wherein in theimpurities, the P content is 0.050% or less and the S content is 0.050%or less, (b) subjecting the prepared austenitic stainless steel to asolid solution heat treatment at a temperature of 1000 to 1200° C., (c)subjecting the heat treated austenitic stainless steel to a cold rollingin which the reduction of area is 20% or more, and (d) subjecting thecold-rolled austenitic stainless steel to a heat treatment in thetemperature range of 900° C. or higher and lower than the solutiontreatment temperature.
 2. The method of manufacturing an austeniticstainless steel according to claim 1, characterized in the step (c)being subjecting the heat treated austenitic stainless steel to a coldrolling in which the reduction of area is 20 to 90%.
 3. The method ofmanufacturing an austenitic stainless steel according to claim 1,characterized in the step (d) being subjecting the cold-rolledaustenitic stainless steel to a heat treatment in the temperature rangeof 900 to 1150° C.
 4. A method of manufacturing an austenitic stainlesssteel for hydrogen gas, wherein the tensile strength is 800 MPa orhigher, a relative rupture elongation is 80% or more, the grain sizenumber in accordance with ASTM E112 is No. 8 or higher, and alloycarbo-nitrides having a maximum diameter of 50 to 1000 nm are containedin the number of 0.4/μm² or larger in cross section observation,comprising the following steps (a) to (d): (a) preparing an austeniticstainless steel that has the chemical composition comprising, by masspercent, C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more to less than7%, Cr: 15 to 24%, Ni: 10 to 15%, Al: 0.001 to 0.10%, N: 0.10 to 0.50%,and V: 0.01 to 1.0%, further containing Nb: 0.01 to 0.50%, and/orcontaining one or more elements of at least one group selected fromelement groups of a first group to a fourth group described below, thebalance being Fe and impurities, wherein in the impurities, the Pcontent is 0.050% or less and the S content is 0.050% or less: firstgroup elements . . . Mo: 0.3 to 3.0% and W: 0.3 to 6.0%, second groupelements . . . Ti: 0.001 to 0.5%, Zr: 0.001 to 0.5%, Hf: 0.001 to 0.3%,and Ta: 0.001 to 0.6%, third group elements . . . B: 0.0001 to 0.020%,Cu: 0.3 to 5.0%, and Co: 0.3 to 10.0%, fourth group elements . . . Mg:0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, La: 0.0001 to 0.20%, Ce:0.0001 to 0.20%, Y: 0.0001 to 0.40%, Sm: 0.0001 to 0.40%, Pr: 0.0001 to0.40%, and Nd: 0.0001 to 0.50%, (b) subjecting the prepared austeniticstainless steel to a solid solution heat treatment at a temperature of1000 to 1200° C., (c) subjecting the heat treated austenitic stainlesssteel to a cold rolling in which the reduction of area is 20% or more,and (d) subjecting the cold-rolled austenitic stainless steel to a heattreatment in the temperature range of 900° C. or higher and lower thanthe solution treatment temperature.
 5. The method of manufacturing anaustenitic stainless steel according to claim 4, characterized in thestep (c) being subjecting the heat treated austenitic stainless steel toa cold rolling in which the reduction of area is 20 to 90%.
 6. Themethod of manufacturing an austenitic stainless steel according to claim4, characterized in the step (d) being subjecting the cold-rolledaustenitic stainless steel to a heat treatment in the temperature rangeof 900 to 1150° C.